Existing ultrasonic pulse-echo techniques rely principally on a comparison of the amplitudes of the signal and noise responses, commonly called the signal to noise ratio, for the proper identification of a flaw. Reliability of defect detection, therefore, requires this signal-to-noise ratio to be substantially greater than unity. The present invention provides additional distinguishing characteristics of flaw and noise responses, namely, motion, which permits defect identification under conditions of unfavorable signal-to-noise ratios. In a typical embodiment the transducer is continuously rotated about its longitudinal axis as it scans the surface of the material being inspected. The result is a dynamic display on the face of the cathode ray tube of the pulser/receiver where the apparent motion of the noise responses differ from that of a flaw.
One of the most vexing limitations to the ultrasonic pulse-echo inspection method is imposed by spurious signals that accompany the response from a true material flaw. In order to identify a flaw against this background, it is necessary that the amplitude of the flaw signal be substantially greater than those of the spurious signals, commonly called noise. That is, the signal-to-noise ratio must be favorable by exceeding unity an amount dependent on the skill and experience of the particular operator. A highly skilled operator may sometimes be able to use other characteristics than amplitude for flaw identification. The flaw of interest may have a particular shaped response or the duration of its response, as the surface is scanned, may be longer than the accompanying noise responses. These situations, however, are too rare and subjective to be considered a practical solution to the unfavorable signal-to-noise ratio problem.
Noise may be considered to be of two types which for our purpose can be called electronic and acoustic. Electronic noise can originate internally from the inspection equipment or from some external radiating source. It is characterized by being random or time independent and by having a high frequency content. Improved circuitry design, instrument construction, proper shielding, the use of solid-state components, and selective filtering have virtually eliminated electronic noise as a problem. Only under inspection conditions requiring very high gain, when it appears as "grass" along the base line, does it become a factor of any significance. Signal processing involving signal averaging and selective filtering can be used to improve the reliability in the above case.
Acoustic noise, however, has the same characteristics as the response from an actual flaw. Because it is composed of true reflections, it is time dependent and has the same frequency content as the flaw response. Time dependence means that if the transducer remains stationary, the displayed noise responses will not change position, shape or amplitude. They are also repeatable each time the transducer is returned to the same location. Because of the above characteristics, the techniques used to reduce electronic noise are not applicable. (Rothman, Eugene, "Electronic Signal Processing Techniques", Phase I and II, ARPA Order No. 1246, Contract No. DAAA25-69-C-0206 (July, 1969, and May, 1970).)
Each noise response is composed of numerous reflections from small discontinuities randomly scattered throughtout and/or on the surface of the material under inspection. These reflections return by different paths and those arriving the same time at the transducer are summed by the instrument and displayed as a single response. Each noise response therefore is the result of a particular group of small reflectors. These reflectors are caused by material features such as grain boundaries, precipitates, surface roughness, or any other points of abrupt change in acoustic impedance. Although the characteristics of the noise and flaw responses are the same. there is a difference in the nature of their source. Whereas the noise response is from a group of randomly scattered reflectors, the source of the flaw response, practically speaking, is a definite intercepted area and a fixed location in the part.
Investigations have been conducted by others where signal processing methods were used to increase an unfavorable signal-to-noise ratio to a value where the flaw response can be identified. One method consisted of adding the amplitudes of the response signals coming from a plane a fixed distance below the top surface of the material, for a discrete time interval, as the transducer moved along at a constant linear speed. Since during this time interval as the amplitude of a flaw response was more stable than any noise response, the resulting summation of the flaw response was substantially greater than any noise response. In effect, the signal-to-noise ratio was increased. (Ermolov, I. N. and Pilin, B. P., Possible Ways of Improving the Sensitivity of Ultrasonic Inspection of Parts with a Large-Grain Structure, Soviet Journal of Nondestructive Testing (Jan-Feb, 1969) Vol 1, pp46-49.)
Another method consisted of linearly scanning a plane a fixed distance below the surface of the material with an array of transducers positioned at different incident angles. The responses of the transducers were multiplied by each other. Since the amplitude of the flaw responses were more stable than any noise responses, the product of the former was substantially greater than the latter. Again, the result was an increase in the signal-to-noise ratio. (Kennedy, J. C. and Woodmansee, W. E., "Electronic Signal Processing Techniques," Phase III, ARPA Order No. 1246, Contract DAAA 25-69-C-0206 (Jan, 1972).)
A current investigation is being conducted with the objective of enhancing the flaw response by signal averaging a number of wave form responses as the transducer oscillates horizontally while scanning the material. (Regalbuto. J. A., "Nondestructive Testing of Diffusion-Bonded Titanium Alloys for Engine and Airframe Components," First Semiannual Interim Technical Report, Air Force Materials Laboratory Contract F33615-72-C-1705 Project 7351 (June, 1972).)